1. Field of the Invention
This invention relates to a process for fabrication of a semiconductor device, particularly to a process for forming electrodes or wirings by utilization of the selective growth of a metal or metal silicide.
2. Description of the Related Art
FIG. 1 is a sectional view of the pertinent portion of a bipolar transistor proposed in the prior art. In this Figure, 1 is an n-type silicon substrate for the collector region, 2 is its principal surface, 3 is a p-type base region, 4 is an n-type emitter region, 5 is a base taking-out electrode (base contact) of a polycrystalline silicon layer containing a p-type impurity, 6 and 7 are SiO.sub.2 films, 8 is a base electrode, and 9 is an emitter electrode. In this bipolar transistor, the base electrode 8 is connected to the base taking-out electrode 5 of a polycrystalline silicon. Such a structure can make the area on the principal surface 2 of the base region 3 smaller as compared with the case when the base electrode 8 is connected directly onto the base region 3 without the intermediary base taking-out electrode 5.
However, even in this bipolar transistor, since the base taking-out electrode 5 consisting of a polycrystalline silicon layer is commonly formed by etching a polycrystalline silicon layer formed on the entire surface of the base region 3, the emitter region 4, and the SiO.sub.2 film 6, the area occupied by the base taking-out electrode 5 on the base region 3 becomes relatively larger. Also, the area occupied by the SiO.sub.2 film 7 on the regions 3 and 4 cannot help but become relatively larger for the same reason. Accordingly, there has been proposed a process for fabricating a bipolar transistor having solved these problems by self-alignment utilizing the good workability of polycrystalline silicon (Japanese Patent Publication (Kokoku) Nos. 55-26630 and 57-32511).
It is also known in the art that silicon, tungsten, etc., can be grown selectively only on the region comprising a specific substance of the substrate. As an application of this selective growth for a semiconductor device, it has been proposed, for example, to form an insulating film selectively on a silicon semiconductor substrate, followed by selective epitaxial growth of silicon only on the portion where no insulating film is formed on the silicon substrate, and utilize the selective epitaxial growth region as an active region for the formation of elements and the above insulating film as an element separating region. Also, it has been proposed to embed silicon in an electrode contact window opened in an insulating film on a silicon semiconductor body by a selective growth of silicon in the window to make the top surface of the silicon the same height as the top surface of the insulating film, followed by the formation of a wiring on the insulating film and the thus embedded silicon, thereby attempting to flatten the multi-layer wiring.
Further, although different from the selective growth method, in order to produce a symmetric bipolar transistor, it is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-34365 to form an insulating film on a single crystalline silicon substrate, further form a thin polycrystalline silicon film thereon, followed by formation of an opening portion at the region for formation of an active region by etching the polycrystalline silicon film and the insulating film, and forming a silicon region continuous from the silicon substrate through the above opening portion to the insulating film by a simultaneous growth of silicon on both the single crystalline silicon substrate below the opening portion and on the polycrystalline silicon film (Polycrystalline epitaxially deposited silicon). The silicon on the insulating film is thereafter subjected to patterning by etching.
In the above-mentioned bipolar transistor, the base taking-out electrode consists of a polycrystalline silicon because polycrystalline silicon has an excellent workability and enables fine patterning and because it has an excellent adaptability for the self-aligning step. However, although electroconductivity may be imparted to polycrystalline silicon by the introduction of impurities at a high concentration, electroconductivity is still poor when compared with metals, etc., and therefore, a problem arises in that the base resistance becomes somewhat greater; refer to the example of the above bipolar transistor. Also, because it is necessary to introduce an impurity at a high concentration in order to make polycrystalline silicon electroconductive, the impurity will be diffused during subsequent heat treatment from the electroconductive polycrystalline silicon to the semiconductor substrate in contact therewith, whereby a problem arises in that the impurity always becomes highly concentrated in this region to sometimes give undesirable effects in element characteristics.
On the other hand, a metal is highly electroconductive and generally excellent as electrodes or wiring materials, but does not have a satisfactory workability for such as etching, or controllability, and accordingly, it can be used only with difficulty for a precise process such as the fabrication of the bipolar transistor as described above.